The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.
In low-output-voltage dc-dc converters, synchronous rectifiers (SR) are widely used to reduce rectifier conduction loss and improve converter efficiency. However, during a switch's transition, an SR's parasitic body diodes unavoidably carry load current decreasing conversion efficiency because a voltage drops across an SR body diode is much higher than in the switch. In addition, the SR body diode's reverse recovery increases switching losses and electromagnetic interference (EMI).
Today's powering requirements demand fast transient response and high power density and as a consequence converter switching frequencies are increased, resulting in increased switching loss.
Recently, soft-switching techniques have been developed to attempt to reduce switching losses and EMI noises. In particular, zero voltage switching (ZVS) techniques have been used for MOSFET-type switches. ZVS multi-resonant converters (MRC) utilize major parasitic characteristics of the power stages. Typically, all semiconductor devices in an MRC operate with ZVS substantially reducing the switching losses and noise. Quasi-resonant converters (QRC) have been used to overcome the disadvantages of conventional pulse-width modulation (PWM) converters operating at high switching frequencies. QRCs achieve this through ZVS for an active switch and zero current switching (ZCS) for a rectifier diode. However, the switches in both QRCs and MRCs must withstand high voltage stress or high current stress. These stresses restrict the applications of QRCs and MRCs.
Reducing a filter inductance in conventional PWM converters, a ZVS quasi-square-wave (QSW) technique is known to offer ZVS for both active and passive switches without increasing the switches's voltage stress. However, QSW converters suffer high current stress in components resulting in significant conduction losses and forcing the active switches to turn off at high currents.
A similar concept to QSW includes an LC cell in parallel with either the active switch or the rectifier diode, depending on the type of converter. The LC cell typically includes a small inductor, Lr, in series with a large capacitor Cc. The LC cell's high inductor current ripple achieves ZVS turn-on for the active switch. However, the LC cell's current ripple in inductor Lr may be more than twice the ripple in the filter inductor, and the associated conduction and turn-off losses increase significantly.
To achieve ZVS while preserving the advantages of the PWM technique, hybrid topologies are known to incorporate a PWM technique along with resonant converters to minimize circulating energy, conduction loss, and switching loss. Adding an auxiliary switch across the resonant converter in a ZVS-QRC derives a ZVS-PWM converter, which can be considered hybrid circuits of ZVS-QRCs and PWM converters. In these hybrid designs ZVS is typically achieved for the active (power) switch and the converter operates at a constant swathing frequency. However, the power switch is subjected to high voltage stress proportional to the load.
Compared with ZVS-PWM converters, known zero-voltage-transition PWM (ZVT-PWM) converters may be more desirable because soft switching is achieved without increasing switch voltage and current stress. By adding an auxiliary shunt network to discharge switch junction capacitance and shift the rectifier diode current, ZVS is achieved for switch and reverse recovery of the rectifier diode is attenuated, though not eliminated.
In recent years, synchronous rectification has been widely used in low-voltage applications. It is also desirable to use synchronous rectification with higher voltage levels since today's high-voltage MOSFET on-resistance is continually being reduced such that a voltage drop across the MOSFETs are comparable with that of fast-recovery diodes. However, the reverse recovery of a MOSFET's body diodes is a barrier to SR higher voltage applications. For example, SRs with 200V and higher ratings are typically not found in such applications, because the SR body diode's reverse recovery becomes significantly worse as the voltage rating increases; this significantly increases switch and body diode switching losses and the reverse recovery related EMI noise may lead to converter malfunction.
It is also known to reduce rectifier reverse-recovery-related losses in high-voltage boost converters, which can be applied to applications with SRs replacing diodes. However, these techniques only provide a compromised solution since the reverse recovery of diodes is attenuated instead of eliminated.
Therefore, there is a need for a high switching frequency switching ZVS dc-dc converter using an SR, while eliminating body diode conduction loss and reverse recovery loss.